High sensitivity test system for the colorimetric determination of specific gravity or total dissolved solids in aqueous samples

ABSTRACT

A highly sensitive and convenient test method, composition and device for the facile calorimetric determination of total dissolved solids in an aqueous sample having a low specific gravity is presented. The reagent composition comprises the mixture of a complex of a positively charged polyelectrolyte and a negatively charged indicator material, and an appropriate buffer system for maintaining the test system environment at an exact pH depending upon the indicator material selected. A preferable embodiment of this test system comprises the incorporation of the test composition into or with a solid state matrix material such as bibulous paper. The test system is particularly useful in the field testing of potable and recreational waters.

FIELD OF THE INVENTION

[0001] The present invention relates to a facile and utilitariancolorimetric test method, composition and device for the determinationof specific gravity or total dissolved solids in low specific gravityaqueous samples. Because Of its ability to measure low specific gravityfluids, it is primarily directed to potable and recreational watersamples such as those found in swimming pools, spas as well as naturalwater environments. It may however be advantageously used fordetermining specific gravity in other aqueous substances such asbiological and other fluids containing ionic constituents.

[0002] The system is basically a field test but may have utility inanalytical laboratories for screening and other applications requiringimmediate results. The system comprises a composition and method that inits preferable format utilizes a carrier or matrix for retaining thetest reagent and advantageously bringing the active ingredients intocontact with the water sample to achieve a calorimetric readout result.

BACKGROUND OF THE INVENTION AND PRIOR ART

[0003] Total Dissolved Solids (TDS) is a term of art used extensively inthe water quality area. The term describes itself very aptly—it is thetotal amount of inorganic and/or organic substances dissolved in a watersample. It is often described as the “total filterable residue” of awater sample since it is what remains in the water sample after thesuspended or insoluble particulate materials are removed therefrom by astandardized filtration process. Potable and recreational waters usuallycontain primarily inorganic cationic species such as calcium, magnesiumand sodium salts and equivalent amounts of anionic species such aschlorides, sulfates and carbonates.

[0004] Historically, TDS has been determined using either gravimetricprocedures or estimated by using electrical conductivity measurements.The gravimetric procedure is commonly a laboratory methodology since itusually involves precisely determining the residue remaining (by weight)after drying the sample using standardized drying procedures. Electricalconductivity measurement methods are usually easier to utilize; however,they require a dedicated conductivity meter and commonly requireextensive calibration and maintenance practices. They provideestimations of total dissolved solids from the measured conductivity ofthe dissolved ionic species.

[0005] The significance of TDS in determining water quality stems fromthe fact that high TDS can result in taste problems in potable waterarea and from chemical balance problems in the recreational water area.

[0006] More recently, in the medical area, methods have been discoveredand developed to measure the specific gravity of body fluids usingcalorimetric procedures. Generally speaking, the samples being studiedin this area are usually body or other biological fluids and have aspecific gravity much higher than those found in the water quality area.

[0007] The term “specific gravity” is commonly used in the medical areaas opposed to the term “total dissolved solids” as used in the waterquality area. Numerous other terms similar to specific gravity are alsoused in the medical area. Terms such as “specific density”, “ionicstrength”, “divalent cation strength”, “osmolality”, “nosmolarity”, “ionconcentration” and “osmotic pressure” are commonly encountered. Each ofthese terms has itself a specific chemical meaning and definition butfor the purposes of describing medical test systems, the somewhatgeneric term “specific gravity” will be used. The specific gravity of anaqueous sample can be defined as the ratio of its weight to that of anequal volume of pure water.

[0008] The calorimetric analytical schemes used to determine thespecific gravity of a body fluid in the medical area basically utilize apolyelectrolyte and an indicator means capable of creating a detectablecolor response resulting from an ion exchange between thepolyelectrolyte and the ions in the aqueous sample. This color responseis then correlated to specific gravity.

[0009] Polyelectrolytes are usually proprietary polymeric materialshaving pendant ionic groups. They are well known in the art and are usedextensively in chemical ion exchange reactions requiring separation orremoval of ionic species.

[0010] Human urine is the most common body fluid using these analyticalschemes to determine specific gravity and its importance resides in itsuse to diagnose a situation involving electrolyte imbalance and itsassociated diseased states.

[0011] Prior to the development of these calorimetric methods fordetermining the specific gravity of body fluids, clinical chemistrymethodologies employed cumbersome and utilized delicate instruments suchas refractometers and other specialized devices.

[0012] It should be noted here that in addition to having differentterminologies, the specific gravity of a body fluid is quite differentfrom the specific gravity or TDS of a water sample. Body fluids such asurine usually have a range of from about 1.005 to 1.030 specific gravitywhich is equal to a TDS ppm range from 7,000 to 43,000. In contrast,recreational waters typically have a TDS ppm range of values from only300 to 5,000. Further, potable waters have even lower TDS values of fromabout 100 to 1,000 ppm.

[0013] Because of the range differences noted above, it has been foundthat the traditional colorimetric methods used in the medical area wereincapable of detecting the very low specific gravity ranges found in therecreational and potable water area.

[0014] In contrast to these calorimetric prior art methodologies, it hasbeen found that the present test composition, as will be describedlater, involves a very different reaction mechanism.

[0015] It should also be noted that while the term specific gravity issimply a comparison ratio and has no dimensional tag, the term TDS isusually reported as milligrams per liter (mg/L) or parts per million(ppm).

[0016] The prior art surrounding calorimetric specific gravity tests isboth extensive and complicated. Most of this prior art involves patentpublications and in an attempt to present at least representativepatents involving this methodology, the following table is presented:TABLE Exemplary Prior Art Patent or Positive Negative ApplicationComponent Component Comments Present Strongly basic Dye that bindsStrongly Buffered, Invention polyelectrolyte to or Complexes Responds toor anion with the monovalent and exchange positive body divalent ionspolymer Does not work by pH shift Three to ten times more sensitive thanurine SG tests U.S. Pat. No. Benzethonium Ions from buffer Must have5,858,788 chloride and benzethonium Habenstein optional other chloridein quaternary formulation. compounds U.S. Pat. No. Dye that bindsStrongly acidic Uses a strongly 5,403,744 to the negativepolyelectrolyte acidic Zimmerle body polyelectrolyte. Buffered at pH 3or less. Does not work by pH shift in the case of the metachromatic dyeU.S. Pat. No. Acid (H+) Weakly acidic Requires titration 4,318,709polyelectrolyte by acid or base to Falb et al. 75-90% of equivalence.Weakly basic Base (OH—) polyelectrolyte U.S. Pat. No. Weakly basicStrong organic Requires titration 4,473,650 polyelectrolyte acid in therange 20-60% Wang U.S. Pat. No. Strong organic Weakly acidic Requirestitration 4,532,216 base, e.g. polyelectrolyte up to 50% Wang R₄N+ U.S.Pat. No. Strongly basic Ions from buffer Works by pH shift 4,376,827polyelectrolyte Stiso et al. Ions from Strongly acidic Works by pH shiftbuffer polyelectrolyte

[0017] As stated above the table, this listing is only representativeand is being given to present a small window to the plethora ofcombinations and permutations surrounding the essential components usedin current calorimetric specific gravity tests.

SUMMARY OF THE INVENTION

[0018] The method, composition and device of the present inventioninvolves a highly sensitive reagent system for determining the specificgravity or total dissolved solids of an aqueous sample. The basic systemcomprises a. a complex of a positively charged polyelectrolyte polymer(PCPP^(n+)) and a negatively charged indicator material (I⁻) and b. abuffer system sufficient to maintain the mixture at an exactpredetermined pH during the contact and reaction thereof with theaqueous sample. This mixture or composition has been found to generate acolor change in a low specific gravity sample depending upon theconcentration of anions in the water sample.

[0019] To achieve this desired low range detection sensitivity thereagent components were chosen so as to contribute minimally to abackground TDS response. A standard color chart or graph is thenprepared by using a series of known TDS concentration samples anddetermining the color produced with the test method or device. Finally,the TDS value of an unknown sample is determined by comparison of thedeveloped color with the standard color chart or instrumentally readingthe color in a standardized reflectance calorimeter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIGS. 1A, 1B and 1C represent various conditions and responses ofthe present test system to TDS as described in the Examples that follow.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] While certain of the basic building blocks of the presentinventive test composition for determining TDS are similar to those usedin the prior art medical test systems, the present system differs inthat it is essentially a binding or complexing phenomenon between apositively charged polyelectrolyte polymeric (PCPP^(n+)) material andthe negatively charged indicator material.

[0022] When the bound indicator contacts anions in the water sample,binding reversal occurs to release indicator into the reaction mixturecausing the appearance of a color change. Since pH change is not acontributory factor in the test system, a strong buffer is employed toretain the reacting mixture at an optimized test reaction value. As usedherein, the term color change means both the change in intensity of asingle color and the change of one color to another.

[0023] The three necessary constituents of the present inventionaccordingly are 1. a positively charged polyelectrolyte polymericmaterial (PCPP^(n+)), 2. a negatively charged indicator material (I⁻)and 3. a buffer material to maintain system pH at or below the pHindicator pKa.

[0024] The polyelectrolyte component of the present invention is apositively charged water-soluble substance having the capability ofcomplexing with a negatively charged pH indicator as described in thefollowing reaction 1:

[0025] Reaction 1—Formation of Reagent Complex

nIH+[PCPP ^(n+) X ⁻ _(n) ]→nH ⁺ +nX ⁻ +[PCPP ^(n+) I ⁻ _(n)]

[0026] wherein X⁻ is OH⁻ or Cl⁻.

[0027] Examples of the positively charged polymeric materials which havebeen found to be operable in the present invention are:

[0028] poly(4-vinylbenzyltrimethylammonium hydroxide);

[0029] poly(4-vinylbenzyltrimethylammonium chloride);

[0030] poly(diallyldimethyl ammonium) hydroxide;

[0031] poly(diallyldimethylammonium chloride);

[0032] poly[bis(2-chloroethyl)ether-1,3 bis[3-(dimethylamino)propyl)urea]; and,

[0033] poly[oxyethylene(dimethylimino)-ethylene-(dimethylimino)ethylenedichloride]. These materials are polyelectrolytes with a positive chargeon the nitrogen atom.

[0034] The second essential component of the present test system is theindicator material. These are primarily negatively charged pH sensitivedyestuffs capable of binding to the PCPP^(n+) material, as shown inreaction 1 above. These color-generating substances preferably havetransition intervals of from about pH 4.0 to pH 9.0. Exemplary of theindicator materials that can be used in the present reagent system aretriphenylmethane or sulfonephthalein dyes such as thymol blue, m-cresolpurple, xylenol blue, cresol red, phenol red, bromothymol blue andchlorophenol red.

[0035] The third essential component of the present reagent systemcomprises a buffer system of sufficient strength and/or concentration tomaintain the test system at an exact pH but does not interfere with orcontribute to the TDS response. Since the present test system depends onbinding reversal rather than a pH change, however slight or small, thebuffer system must be capable of maintaining the entire mixture of testcomposition and test sample to an exact predetermined pH. This pH isdependent upon the color transition range of the indicator material. Thepreferred buffer components must not have a significant effect on thetest sensitivity.

[0036] Some buffer systems found to be suitable have components asfollows: the base of the buffer system may be selected from the groupconsisting of imidazole; tris(hydroxymethyl)aminomethane;2-amino-2methyl-1,3-propanediol; bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane; and,1,3-bis[tris(hydroxymethyl)methylamino]propane, and the acid selectedfrom the group consisting of carboxylic acids, and preferablymonocarboxylic acids, such as glycolic, lactic, benzoic and aceticacids. This list is obviously representative and not intended to be alimitation to the selection of suitable buffers.

[0037] As previously stated, the reaction mechanism of the presentinvention involves the use of a complexed indicator material andpolyelectrolyte polymer wherein the complexed indicator is released bycontact with anions in the aqueous sample. Referring now to the reactiondescribed in “Reaction 1” above, when thymol blue is used as anindicator (I), the uncomplexed form of this indicator is yellow at itsacidic side pKa and blue at its basic side. In this reaction, the bulkpH of the mixture is maintained at the acidic or yellow side of theindicator pKa. Surprisingly, the [polymer/thymol blue] complexedindicator is now a blue color, even though the bulk pH is still at theacidic side of the indicator pKa.

[0038] Further, it was found that the anionic species (A⁻) of a TDSsample can replace the negatively charged indicator from [PCPP^(n+) I⁻_(n)] complex as shown in Reaction 2 next below.

[0039] Reaction 2. TDS Detection Using Thymol Blue as I⁻

[PCPP ^(n+) I ⁻ _(n) ]+nA ⁻ +nH ⁺ →[PCPP ^(n+) nA ⁻ ]+nIH

[0040] (BLUE) (YELLOW)

[0041] In this reaction the complexed indicator material, thymol blue,is displaced by anions and becomes protonated. It accordingly returns toits yellow uncomplexed form in the bulk phase. In the absence of the TDSanionic species the complex color is blue. With increasing TDS, that is,with increasing concentrations of anionic species, more of theuncomplexed indicator is formed. The increasing yellow form causes acolor change from blue to green to yellow.

[0042] The concentration of the various components used in the presenttest system is of course dependent upon the individual component used;however, generally speaking the following ranges of concentrations ofcomponents may be used: PCPP^(n+) 0.10-30 g/L Indicator Material 0.01-3g/L Buffer 0.30-75 mN

[0043] Other inactive additives, such as thickening agents, stabilizersand surfactants, may also be used in the present test composition toachieve the desired format for presentation of the test system to thetest sample.

[0044] Although the test reagent may be used as a liquid system, aparticularly preferable and advantageous format for the present testcomprises the incorporation of the test reagent components into a matrixfor holding the reagent in a dry, so-called solid state system, untilpresented to the aqueous test sample. When this is done, the reagentrehydrates, reacts with the anions in the test sample and a color isdeveloped in or on the matrix that can be compared to standardized testcolor chart. The matrix may be bibulous paper, a synthetic polymericmaterial or other membrane materials that in turn may be attached tomore rigid plastic sheet materials which acts as a handle for ease ofuse of the test device.

[0045] Such devices are known in the art as reagent strip tests and areusually read visually or the color developed may be read and interpretedby an instrumental means such as a reflectometer. Likewise, if a liquidsystem is employed, the color developed may be read by visual comparisonto standard color tubes or by a calorimeter.

[0046] Since the color produced in the reagent system relates to ionicspecies in the test sample rather than TDS directly, the color must becompared to standardized test samples which have been prepared usinggravimetric procedures. A test comparison chart is then prepared whichcreates a direct correlation of color to TDS.

EXAMPLES Example 1 This Example Describes a Liquid Reagent CompositionShowing Color Change due to Indicator Binding With PCPP.

[0047] A test composition having thymol blue as a pH indicator,poly(4-vinylbenzyl trimethyl-ammonium hydroxide)-(PVBA ) as a positivelycharged polyelectrolyte polymer, tris(hydroxymethyl)aminomethane(THAM)+glycolic acid as a pH buffer system was prepared by mixing thecomponents in distilled water in the amount shown in Table 1. PVBAhydroxide was prepared from commercially available poly(4-vinylbenzyltrimethyl-ammonium chloride) using strongly basic anion exchange resin.Glycolic acid (2 M solution) was added in the amount needed to establisha desirable pH of the composition. TABLE 1 Liquid reagent compositionand properties Reagent Composition No. 1 2 3 Reagent Composition WithoutWithout With And Characteristics Polymer Polymer Polymer Bulk pH 7.4 9.47.4 PVBA mg/L 0 0 150 (Polymer) Thymol blue mg/L 50 50 50 (Indicator)THAM g/L (buffer) 1.8 1.8 1.8 Glycolic acid + + + (pH adjustment) Colorand Yellow Blue Blue Indicator form Uncomplexed Uncomplexed Complexed

[0048] The color response of these compositions is shown in FIGS. 1A and1B. In FIG. 1A, the spectra of the uncomplexed indicator, thymol blue,at pH 7.4 (yellow) and pH 9.4 (blue) are shown (curves 1-2). In FIG. 1B,the spectra of the uncomplexed and complexed indicator, both at pH 7.4,are shown (curves 3-4). Note that the complexed indicator is blue at pH7.4.

Example 2 This Example Shows how Increasing TDS Incrementally Convertsthe Blue Complexed Form of Thymol Blue to the Yellow Uncomplexed Form atpH 7.4.

[0049] Increasing amounts of TDS, as sodium chloride were added to thecomposition No. 3 of Example 1. Color of the composition was changedfrom intense blue to yellow-greenish in the 70-2000 ppm NaCl range.Light absorption was measured. The absorbency of the composition atdifferent increasing TDS levels is shown in FIG. 1C (curves 5-9).

Example 3 Test Strip (Device) Preparation

[0050] A test composition was prepared by dissolving the chemicalslisted in Table 2 below in distilled water. TABLE 2 Reagent compositionfor dry emistry test device Chemical Amount (g/L) Thymol blue, sodiumsalt 0.4 PVBA hydroxide 4.5 Imidazole 3.4 Glycolic acid A sufficientamount to adjust pH to 7.5

[0051] Ahlstrom filter paper was impregnated with the reagent and dried.The test paper was cut into small pads of ⅕ in by ⅕ inch and attached toone end of strips of rigid plastic sheet material approximately ⅕ in.wide by 3 inches long. The prepared test strips were activated for 1second by immersing in standard TDS solutions. Test pad color was readafter 15 seconds. The color changed from blue to yellow-green in the0-5000 ppm TDS range. Reflectance of each activated test pad wasmeasured with a reflectance spectrophotometer. Results are shown inTable 3. TABLE 3 TDS test device dose response TDS ppm 0 440 1400 28804880 as NaCl % Reflectance 7.59 10.62 16.50 23.60 29.06 at 610 nm

Example 4 This Example Demonstrates TDS Test Device Response toDifferent Ionic Species

[0052] The procedure of Example 3 was used to prepare test strips.Sample solutions of 1400 ppm of NaCl, CaCl₂, Na₂SO₄ and MgCl₂ wereprepared and measured with test strips. A one second dip time and a 15second read time were used. The color developed was compared with acolor chart that was made using standard TDS solutions at 0, 440, 1400,2880 and 4880 ppm NaCl. A color chart was prepared to permitsemiquantitative measurement of TDS samples with the test strip. Colorswere selected to match standard TDS concentrations and assigned thenumbers to the colors. The numerical values and corresponding TDS levelsare as follows: 10 (0 ppm), 20 (400 ppm), 30 (1400 ppm), 40 (2880 ppm)and 50 (4880 ppm). Using this chart, the test device response isexpressed in numerical response as shown in Table 4. TABLE 4 TDS testdevice response to different ionic species of samples TDS Sample, TDSGravimetric TDS Test Device Result Ionic Species Sample, ppm Color ChartDesignation NaCl 1400 31 CaCl₂ 1400 32 Na₂SO₄ 1400 32 MgCl₂ 1400 33

What is claimed is:
 1. A method for the determination of total dissolvedsolids in an aqueous sample having a low specific gravity, the methodcomprising a. mixing the sample with a test reagent compositioncomprising a complex of a positively charged polyelectrolyte polymer anda negatively charged indicator material and a buffer system formaintaining the mixture at an exact predetermined pH, the mixturegenerating an incremental color change therein depending upon theconcentration of anions in the water sample, b. determining theresulting color of the mixture and, c. correlating the color of themixture to the concentration of total dissolved solids in the watersample.
 2. A method as in claim 1 wherein the positively chargedpolyelectrolyte polymer is selected from the group consisting of:poly(4-vinylbenzyltrimethylammonium hydroxide);poly(4-vinylbenzyltrimethylammonium chloride); poly(diallyldimethylammonium) hydroxide; poly(diallyldimethylammonium chloride);poly[bis(2-chloroethyl)ether-1,3bis[3-(dimethylamino)propyl)urea]; and,poly[oxyethylene(dimethylimino)-ethylene-(dimethylimino)ethylenedichloride].
 3. A method as in claim 1 wherein the indicator material isselected from the group consisting of triphenylmethane andsulpfonephthalein indicator materials.
 4. A method as in claim 1 whereinthe indicator material is selected from the group consisting of thymolblue, m-cresol purple, xylenol blue, cresol red, phenol red, bromothymolblue, and chlorophenol red.
 5. A method as in claim 1 wherein the bufferis set to maintain the mixture at a pH selected from the groupconsisting of from below and at the normal pKa of the indicatormaterial.
 6. A method as in claim 1 wherein the buffer system isselected from the group consisting of a base component selected from thegroup consisting of: imidazole; tris(hydroxymethyl)aminomethane;2-Amino-2methyl-1,3-propanediol; bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane; and, 1,3-bis[tris(hydroxymethyl)methylamino]propane, and the acid component is selected from the groupconsisting of glycolic acid, lactic acid, benzoic acid and acetic acid.7. A test composition for the determination of total dissolved solids inan aqueous sample having a low specific gravity, the compositioncomprising a. a complex of a positively charged polyelectrolyte polymerand a negatively charged indicator material and b. a buffer system formaintaining the mixture at an exact predetermined pH.
 8. A testcomposition as in claim 7 wherein the positively charged polyelectrolytepolymer is selected from the group consisting of:poly(4-vinylbenzyltrimethylammonium hydroxide);poly(4-vinylbenzyltrimethylammonium chloride); poly(diallyldimethylammonium) hydroxide; poly(diallyldimethylammonium chloride);poly[bis(2-chloroethyl)ether-1,3bis[3-(dimethylamino)propyl)urea]; and,poly[oxyethylene(dimethylimino)-ethylene-(dimethylimino)ethylenedichloride].
 9. A composition as in claim 7 wherein the indicatormaterial is selected from the group consisting of triphenylmethane andsulfonephthalein indicator materials.
 10. A composition as in claim 7wherein the indicator material is selected from the group consisting ofthymol blue, m-cresol purple, xylenol blue, cresol red, phenol red,bromothymol blue, and chlorophenol red.
 11. A composition as in claim 7wherein the buffer is set to maintain the mixture at a pH selected fromthe group consisting of below and at the normal pKa of the indicatormaterial.
 12. A composition as in claim 7 wherein the buffer system isselected from the group consisting of: imidazole;tris(hydroxymethyl)aminomethane; 2-Amino-2methyl-1,3-propanediol;bis(2-hydroxyethyl)imino tris(hydroxymethyl)methane; and,1,3-bis[tris(hydroxymethyl)methyl amino]propane, and the acid componentis selected from the group consisting of glycolic acid, lactic acid,benzoic acid and acetic acid.
 13. A test device for the determination oftotal dissolved solids in an aqueous sample having a low specificgravity comprising incorporating the dried residue of a liquid mixtureof a test composition comprising a. a complex of a positively chargedpolyelectrolyte polymer and a negatively charged indicator material andb. a buffer system for maintaining the mixture at an exact predeterminedpH with a solid water insoluble matrix material.
 14. A test device as inclaim 13 wherein the matrix material is bibulous paper.
 15. A testdevice as in claim 13 wherein the positively charged polyelectrolytepolymer is selected from the group consisting of:poly(4-vinylbenzyltrimethylammonium hydroxide);poly(4-vinylbenzyltrimethylammonium chloride); poly(diallyldimethylammonium) hydroxide; poly(diallyldimethylammonium chloride);poly[bis(2-chloroethyl)ether-1,3bis[3-(dimethylamino)propyl)urea]; and,poly[oxyethylene(dimethylimino)-ethylene-(dimethylimino)ethylenedichloride].
 16. A test device as in claim 13 wherein the indicatormaterial is selected from the group consisting of triphenylmethane andsulfonephthalein indicator materials.
 17. A test device as in claim 13wherein the indicator material is selected from the group consisting ofthymol blue, m-cresol purple, xylenol blue, cresol red, phenol red,bromothymol blue, and chlorophenol red.
 18. A test device as in claim 13wherein the buffer is set to maintain the mixture at a pH selected fromthe group consisting of below and at the normal pKa of the indicatormaterial.
 19. A test device as in claim 13 wherein the buffer system isselected from the group consisting of a base component selected from thegroup consisting of: imidazole; tris(hydroxymethyl)aminomethane;2-Amino-2methyl-1,3-propanediol; bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane; and, 1,3-bis[tris(hydroxymethyl)methylamino]propane, and the acid component is selected from the groupconsisting of glycolic acid, lactic acid, benzoic acid and acetic acid.20. A test device as in claim 13 wherein the complex comprises thymolblue and poly(vinylbenzyl trimethyl ammonium hydroxide).